The invention relates generally to anti-lock braking systems. More particularly, the invention concerns proportional control of brake fluid pressure modulation in anti-skid braking systems.
Automatically controlled anti-skid braking systems (ABS) traditionally have sought to attain three goals. The first goal of an ABS is to avoid front wheel lockup. Under such a condition, one loses "steerability." the second goal of a typical ABS is to avoid "fish-tailing" or rear-end instability. The third goal of an ABS is to minimize the stopping distance. It has been found that the stopping distance of the vehicle may be made shorter if the wheels are operated at low slip rather than in a fully locked or skid condition (the effective coefficient of friction is greater at lower slip than at full slip).
The typical ABS attempts to optimize stopping distance, steerability, and rear-end stability during so-called "panic stops." In a typical ABS method, one desires a high brake torque "apply" rate for quick response. Additionally, one needs a high "release" rate, if the condition of lockup is sensed as about to begin. The conditions of "apply," "hold," and "release" refer respectively to increase, constant and decreasing brake pressure or resulting brake torque. The apply state means brake torque is being increased, the release state means that brake torque is being decreased, while the hold state indicates that the brake is being maintained constant.
The typical ABS system now utilizes a control computer in conjunction with wheel speed sensors and a stored program ABS algorithm to effect the attainment of the above three goals. One such anti-skid braking control method is disclosed in a copending application to Douglas E. Bernard entitled Anti-Skid Braking System Control Method which is assigned to the same assignee as this invention.
Regardless of the ABS control method employed, the hardware for effecting the apply, hold or release braking commands, i.e., the modulation arrangement for the anti-lock brake system, obviously becomes a key element of any ABS system.
Such hardware approaches to effecting brake pressure modulation are discussed in an article entitled "Hydraulic Brake Actuation Systems Under Consideration of Anti-Lock Systems and Disc Brakes" by Depenheuer and Strien, paper number 730535 presented at the National Automobile Engineering Meeting, Detroit, Mich., May, 1973. A more detailed arrangement is presented in a paper by Schurr and Dittner entitled "A New Anti-Skid Brake System for Disc and Drum Brakes," paper number 840468 presented at the International Congress and Exposition, Detroit, Mich., February, 1984.